[wilun]

The video is typically subsampled at encoding at capture resolution, but it is also subsampled at other encoding resolutions. Because the whole point of subsampling is to be taken into account during encoding, and encoding itself needs not to vary depending on whether the source was downscaled or not.

So video codecs most of the time work with some subsampled chroma components. So your encoded 1080p might be able to render after decoding only e.g. 540 lines of those components, while with the 4k stream it might be: 2160/2 => back to 1080.

Edit: but to be clear, I'm not advocating for people to choose 2x stream and start watching 4k on FHD screens in general, that would be insane. Chroma subsampling is used because the eye is less sensitive to those colors.

[shawn]

I would be interested in a double blind experiment confirming that their specific implementation of chroma subsampling is even detectable. The eye is much less sensitive to colors than intensity, as you point out. If it were perceptible, I think the codec designers wouldn't feel it was an acceptable tradeoff.

So your encoded 1080p might be able to render after decoding only e.g. 540 lines of those components, while with the 4k stream it might be: 2160/2 => back to 1080.

I'm not sure that's accurate -- whatever downscaling process was used to convert from 8k to 1080p on Google's servers is probably the same process to convert from 8k to 1080p in the youtube player, isn't it? At least perceptually.

I would agree that if they convert from 8k (compressed) to 4k (compressed), then 4k to 1080p (compressed), then that would introduce perceptible differences. But in general reencoding video multiple times is fail, so that would be a bug in the encoding process server side. They should be going from the source material directly to 1080p, which would give the encoder a chance to employ precisely the situation you mention.

Either way, you should totes email me or shoot me a keybase message. It's not every day that I find someone to debate human perceptual differences caused by esoteric encoding minutiae.

[verall]

It's not just that the eye is less sensitive to chroma.

Although your 4:2:0 subsampled 1080p video only has 540x960 pixels with chroma information, the decoder should be doing chroma upsampling, and unless its a super simple algorithm it should be doing basic edge detecting and fixing the blurry edges chroma subsampling is known to cause. I posit that even with training, without getting very very close to your screen you wouldn't be able to tell if the source material was subsampled 4:2:0, 4:2:2, or 4:4:4.

The truth is that generally people DO subjectively prefer high resolution source material that has been downscaled. Downscaling can clean up aliasing and soften over-sharp edges.

People who watch anime sometimes upscale video beyond their screen size with a neuron-based algorithm, and then downscale to their screen size, in order to achieve subjectively better image quality. This is even considering that almost all 1080p anime is produced in 720p and then upscaled in post-processing!